1. Field of the Invention
This invention relates to mixing a binder which includes a dispersion medium to metallic or ceramic powders, plasticizing-molding or casting-molding the resulting mixture and eliminating the dispersion medium without causing inconveniences to the resulting molded product.
2. Description of the Prior Art
Heretofore, molded products of complex shape such a small-size machine parts or turbine components are produced in a known manner by resorting to injection molding and by employing metallic powders, such as powders composed of 2% of Ni and 98% of Fe, SUS 316 or sterite powders, powders of ceramics such as alumina, silicium carbide, silicium nitride or zirconia, or a mixture of ceramic powders and powders of metals such as cobalt-tungstene carbide, as starting material.
In general, injection molding is composed of the following steps;
(i) The step of mixing powders of starting materials with a binder including a dispersion medium to give a thermoplastic mixture;
(ii) The step of softening the mixture in a heated cylinder and injecting the thus-softened mixture into a metallic mold;
(iii) the step of opening the metallic mold to take out the molded product;
(iv) the step of de-greasing for eliminating the dispersion medium from the molded product; and
(v) the step of calcination for elevating the density of the molded product.
The success of injection molding consisting of these respective steps depends notably on the kind of binder employed in the process. Above all, the yield of the ultimate products is occasionally influenced by whether the binder employed is suited or not.
The purpose of using the dispersion medium in the binder is to impart plasticity of the starting pulverulent material while improving its moldability. In case of poor moldability, flaws such as silver marks, weld lines or sink marks are produced in the molded products.
While moldability can be improved by increasing the amount of the dispersion medium, a large amount of dispersion medium is naturally removed in the degreasing step with the result that flaws such as crevices, deformation or foaming are caused to exist in the molded products.
In practicing the above described molding process, it has been tried to reduce the amount of the dispersion medium to a smallest value possible while using such dispersion medium as will not cause the aforementioned defects during the degreasing step. In general, a mixture of low molecular weight polyethylene, polystyrene, paraffin or fine crystal wax and a minor amount of oil or thermoplastic resin is used as such dispersion medium.
In addition, polypropyrene, polyvinyl alcohol, polyvinyl butyral, polyethylene glycol, methyl cellulose, atactic ethyl cellulose, hydroxy ethyl cellulose or acrylic resin can also be used as the dispersion medium. It has also been customary to add a small amount of stearates into the dispersion medium for improving mold release properties of the molded product.
It is noted that the following difficulties are presented with these conventional dispersion mediums.
(i) In the above method, the dispersion medium is cracked and vaporized off in the degreasing step by heating to a temperature of ca. 400.degree. to 500.degree. C. At this time, it is necessary that the molded product be gradually heated to a preset temperature so that the gas will be evolved with cracking of the dispersion medium at a rate not higher than the rate of gas diffusion into the molded product. When the rate of gas evolution is higher than that of gas diffusion, the pressure in the molded product is increased thus occasionally causing deformation, cracking or foaming. Therefore, a processing time of 70 to 100 hours is usually required in the degreasing step so that the merit of higher productive efficiency proper to the techniques of injection molding is lost.
(ii) While the heating over an extended period of time is required in the degreasing step, the evolved heat is rather low and hence economically difficult to recuperate as effective heat energy. Thus it is discarded as waste heat.
(iii) Although degreasing is terminated with completion of heat cracking of the dispersion medium, it is not possible to remove the grease completely so that small amounts of carbon or oil and fat are left in the dispersion medium. These residual amounts of carbon or oil and fat may cause injury to the properties of the calcined product.
(iv) Since the products recovered from the degreasing process are decomposition products of the dispersion medium and cannot be reused, they are usually discarded, with the result that production costs are correspondingly elevated.
The aforementiond problems yet to be solved are presented not only in the techniques of injection molding but in the techniques of molding of plastic materials in general, such as those of extrusion molding or die casting.
There is also known a casting-molding method for molding of metallic powders, ceramic powders, or a mixture of metallic and ceramic powders. According to this method, a viscous slime of starting powders is cast into a liquid-absorbing mold to form a wall layer to produce the molded product.
For producing the viscous slime of starting powders, the starting powders are mixed with a minor amount of the binder including aqueous dispersion medium such as water and crushed together in a ball mill. The resulting slime can be stirred for several days, adjusted for viscosity or moisture and defoamed in vacua for improving its stability.
A plaster mold is assembled after the mold release agent is applied on the surfaces of the various mold components. The slime prepared in the above described manner is cast into the mold. With the absorption of moisture into the mold, a wall layer is formed on the mold surface and grown with the lapse of time.
For molding a hollow article, excess slime can be discharged when the wall thickness reaches a predetermined value.
Since the mold into which the slime has been cast as described above continues to absorb the moisture, water contents in the wall layer are decreased gradually so that the cast article is increased in hardness and contracted so as to be detached from the mold.
At this time, the cast article is removed and subjected to rough and finish machining steps, followed by drying. The plaster mold from which the cast article has been removed can be dried for repeated usage.
A variety of alcohols can be used besides water as the dispersion medium used for the preparation of the starting slime of the powder mixture.
As mold materials, water permeative mold materials selected from the group of plastic materials superior in mechanical strength or wear resistancy and the metal-ceramic fiber composite materials can be used in place of plaster for the preparation of the water permeative molds.
There is also known a method according to which a core is inserted while the powdered material is tamped, the molding is carried out with the compacted powder mold being with subsequent removal of the core, the mold being destroyed after temination of molding for returning the compacted powders back into the batch of the powdered material. In this case, the powdered material can be advantageously reused.
The major problem in the casting-molding of the metallic and ceramic powders is presented in the drying process. There are two main steps in the drying process. In the first constant-rate drying step, the water is lost from the surface of the cast article, which then undergoes shrinkage corresponding to the volume of the lost water. In the next reduced-rate drying step, the water is vaporized within the molded product. The shrinkage which the molded product undergoes during this step is substantially nil.
With the cast article with variable thicknesses, it takes some additional time until the portion of the cast article with a large thickness is shifted from the constant rate drying to the reduced rate drying, so that shrinkage does not proceed smoothly. In the case of a molded article with a larger thickness, it takes some additional time until the inner part of the article is dried, with the shrinkage on the superficial portions taking place more promptly than that at said inner portion. In this case, the molded article is likely to undergo strain or crevice formation in the course of the drying process.
In order to prevent strain or cracks from occurring, it is necessary that the inner part of the molded product in its entirety be cooled uniformly by using a lower drying temperature. To this effect, natural drying is possibly most preferred. However, natural drying is influenced by climatic conditions and need be carried out over an extended time while also requiring a large floor space and a lot of man-power.
The drying with the aid of hot air is also inconvenient in that the air volume and velocity, temperature and humidity need be maintained at uniform values, while the adequate control is difficult to achieve because of too many setting elements.
There is also known a high frequency drying method, according to which the drying proceeds relatively uniformly. It is however difficult to dry the molded product without producing heat stresses in the respective portions as well as the inner and outer layers of the molded product. Thus, sporadic drying, crevices, cracks or strain is caused to a more or less degree with the result that the yield of the dried product is necessarily lowered.